Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of substrates particularly those that are soft, flexible, or easily deformed, such as paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, and laminates. Course surfaces and stretchable polymeric films can be economically printed by the means of flexography.
Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for making ink impressions on the printed substrates. The raised relief images are inked in contrast to the relief “floor” that remains free of ink during printing. Such printing plate precursors are generally supplied to the user as one or more layers on a suitable backing or substrate. Flexographic printing is often carried out using a flexographic printing cylinder or seamless sleeve having a desired relief image.
Flexographic printing plates have been prepared in a number of ways. Initially, the images were cut into a sheet of rubber with a knife. An improvement was achieved by forming a mold that could be produced by photo-etched graphics and then by pouring molten rubber or elastomer into the mold and vulcanizing it to form the printing plate precursor. More recently, relief images have been prepared by exposing a photosensitive composition coated onto a substrate through a masking element or transparency and then removing non-exposed regions of the coating with a suitable solvent. Various photosensitive compositions are known for this purpose and usually utilize some type of polymerization, for example, using free radicals.
Direct laser engraving is described in a number of publications including U.S. Pat. Nos. 5,798,202 and 5,804,353 (Cushner et al.) in which various means are used to reinforce the elastomeric layers. Laser-engraveable elements may also include hydrocarbon-filled plastic and heat-expandable microspheres as described in U.S. Patent Application Publication 2003/0180636 (Kanga et al.).
Direct laser engraving innovations often employ the use of carbon dioxide laser or near infrared diodes. In the former case, the radiation of the laser beam is at 10.7 μm and is absorbed by the polymeric materials that are present. In the case of the near infrared imaging sources such as diode lasers, an absorbing material such as a dye or pigment must be present because in general, polymers do not absorb in that part of the spectrum.
Flexographic printing plate precursors that are to be imaged using near infrared ablation need an elastomeric or polymeric imaging layer that is preferably prepared by a polymerization reaction and includes appropriate fillers and infrared radiation (IR) absorbing compounds such as carbon black.
Thermoplastic materials that have not been crosslinked to form a thermoset material have been found to have limited suitability because ablation of thermoplastic materials tends to cause melting of non-ablated regions around the ablated regions and re-deposits ablated debris in the ablated regions.
U.S. Pat. No. 5,278,023 (Bills et al.) describes non-flexographic laser ablation systems that image various elements containing “propellants” that improve decomposition during ablation. For example, the patent describes the use of propellants in laser donor materials to assist the transfer of an image to a suitable receiver material.
Waterless offset printing plates are described in WO 1994/01280 (Gates et al.) in which gas-producing materials (“blowing agents”) such as sulfonyl hydrazide and azodicarbonamide are included to encourage thermal degradation of the thin printing plate layers. These additives are incorporated into suitable decomposable polymers.
As noted above, the use of powerful IR lasers enables higher quality and more reliable engraving compared to the older carbon dioxide lasers. There is a desire to optimize imaging speed or sensitivity by finding better imaging compositions that can be successfully imaged using IR-laser ablation. IR lasers usually require the presence of IR-absorbing dyes or pigments, but if the laser-ablatable layer includes thermoset polymers, they must be polymerized in a manner that is not affected by the IR dye or pigment, or conversely affects the IR dye or pigment. This is a formidable task as the imaging layer is quite thick, for example up to 6 mm. For example, if curing is carried out using free radical chemistry, polymerization can act on the IR dye so that it no longer absorbs in the infrared region. On the other hand, if carbon black or iron oxide is used in place of the IR dye, polymerization of the relatively thick laser-ablatable layer using UV light is extremely difficult.
A number of elastomeric imaging compositions have been formulated for making flexographic printing plates. They have generally been UV-radiation sensitive compositions as evidenced by EP 1,228,864A1 (Houstra) and U.S. Pat. No. 5,798,202 (noted above) and U.S. Pat. No. 6,935,236 (Hiller et al.). UV curing has a number of disadvantages and is difficult to use with relatively thick laser-ablatable layers. While many polymers have been suggested for use in flexographic printing plate precursors, only elastomers are useful in practice because they can be bent around printing cylinders and secured with temporary bonding that fix the plate during printing and can be removed after printing.
As far as printing plates precursors are concerned, the use of “blowing agents” or propellants has been confined to reaction during precursor fabrication (for example, heat, UV, or electron beam curing). Because of their reactivity under free radical conditions, one would not expect such compounds to be useful during laser ablation of such precursors. They may have been used in thin, solvent-deposited layers but the expectation is that they would be useless in thick flexographic laser-ablatable layers especially in the presence of free radicals. For example, WO 2005/084959 (Figov) reports that the presence of peroxide causes “blowing agents” to decompose. Thus, there are reasons given in the art to avoid their use in laser-ablatable flexographic printing plates.
Problem to be Solved
There is a continuing need to provide flexographic printing plate precursors with greater imaging sensitivity. If “blowing agents” or propellants can be used for this purpose, there is a need to have imaging compositions in which such compounds are not prematurely reacted or decomposed before laser ablation.